In predicting the precise times of topocentric phenomena, like solar eclipse contacts, both TTand UT1 come into play. Therefore, assumptions have to be made about the value of ∆T at thetime of the phenomenon. Alternatively, the circumstances of such phenomena can be expressedin terms of an imaginary system of geographic meridians that rotate uniformly about the Earth’saxis (∆T is assumed zero, so that UT1=TT) rather than with the real Earth; the real value of ∆T then does not need to be known when the predictions are made. The zero-longitude meridian ofthe uniformly rotating system is called the ephemeris meridian. As the time of the phenomenonapproaches and the value of ∆T can be estimated with some confidence, the predictions can berelated to the real Earth: the uniformly rotating system is 1.002738 ∆T east of the real systemof geographic meridians. (The 1.002738 factor converts a UT1 interval to the equivalent EarthRotation Angle — i.e., the sidereal/solar time ratio.)

Hence I do: 1.002738 * DeltaT = 68.6s that is a value for which the latest -1 leap second that subtracted from Espenak-Meeus get the same value: 69.6 - 1 = 68.6s. So my question is: is correct to do so or it is just a coincidence? I cannot test the systemtime neither onward nor backwards because it is fixed at "now" but these values should be the same also in simulation (harking back from internal TDB to UTC).

This is Saturn from Rome at 2016-Apr-26 00:00. All the objects have the same issue. The first image show the horizontal grid in VSOP87, the second the azimuth and elevation from JPL HORIZONS and the third the horizonta grid in SPICE, respectively. Alt-Az are topocentric refracted.

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The grid is not where it should be.Note: do not use the current version of the plugin to check altitude; that is a more enhanced new version not yet released.

meanwhile, I further simplified quite significantly the code along with the procedure of investigating mutual (Galilean) events. Only the earth-bound date-time of an event (UTC) needs to be entered into celestia.Sci's Time dialog, the rest proceeds automatically.

Let me show you for the previously addressed two events how well they match the respective predictions.

1) 1ECL2_2003_3_13.jpg++++++++++++++++++++In this event the shadow of Io is partially cast onto Europa. The event happened in March 13 of 2003 with a maximum around 23h 05m.

[Click on image for a bigger size and then hit your browser's fullscreen key (F11?)]

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On the r.h.s you also see the filled-in Time dialog for this example. Note that the Time dialog is opened by simply clicking onto the 1st tool-button in the lower left of the canvas! Very handy...The eclipse event happend just a few minutes before Europa vanishes behind Jupiter, a small part of which you can see on the right...

Dr. Arlot's predicted time (TT=TerestialTime) for the maximum of the event is visible in the simulation settings, and indeed the display verifies the perfect agreement!Couldn't be better...

2) 2O1_2014_12_20.jpg+++++++++++++++++++Here Europa occults Io partially . The predicted maximum is at 05h:36m in perfect agreement with the celestia.Sci simulation!

Here is the corresponding screenshot! This time the Time dialog is dogged into the main canvas on the right. Just to illustrate a few of the many possibilities...

[Click on image for a bigger size and then hit your browser's fullscreen key (F11?)]

After the Jovian system, the focus relies on Pluto-Charon mutual events and most recently: Observed Transits of the EXO planets b , c and d across their star Trappist-1! (Using Andrew's orbit data). viewtopic.php?f=17&t=828#p13862

The latter checks are particularly fun, since the distance amounts to 39.466 ly. Hence celestia.Sci (Celestia) predictions of the transit of EXO b, for example, test a lightTravelDelay of 14415.03 days for EXO b, the orbital period of which is only 1.51085 days!

In other words: an event having taken place in Nov 29 1976 within the Trappist-1 system would be observed on Earth today!

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